
Case 1: t_{xy} > 0 and s_{x} > s_{y} 


The principal axes are counterclockwise to the current axes (because t_{xy} > 0) and no more than 45º away (because s_{x} > s_{y}).


Case 2: t_{xy} < 0 and s_{x} > s_{y} 


The principal axes are clockwise to the current axes (because t_{xy} < 0) and no more than 45º away (because s_{x} > s_{y}).


Case 3: t_{xy} > 0 and s_{x} < s_{y} 


The principal axes are counterclockwise to the current axes (because t_{xy} > 0) and between 45º and 90º away (because s_{x} < s_{y}).


Case 4: t_{xy} < 0 and s_{x} < s_{y} 


The principal axes are clockwise to the current axes (because t_{xy} < 0) and between 45º and 90º away (because s_{x} < s_{y}).


Case 5: t_{xy} = 0 and s_{x} > s_{y} 


The principal axes are aligned with the current axes (because s_{x} > s_{y} and t_{xy} = 0).


Case 6: t_{xy} = 0 and s_{x} < s_{y} 


The principal axes are exactly 90° from the current axes (because s_{x} < s_{y} and t_{xy} = 0).
